1. Field of the Invention
The present invention generally relates to microstrip antennas, and more particularly, to a microstrip antenna having symmetric width discontinuities at a patch portion for enabling reduction in antenna size without sacrificing antenna efficiency too much.
2. Description of the Related Art
Advances in digital and radio electronics have resulted in the production of a new breed of personal communications equipment posing special problems for antenna designers. As users demand smaller and more portable communications equipment, antenna designers are pressed to provide smaller profile antennas. Additionally, users of such communications equipment desire high data throughput, thus requiring antennas with wide bandwidths and isotropic radiation patterns. Moreover, antennas in such portable equipment are often randomly oriented during use, or used in environments, such as urban areas and inside buildings, that are subject to multipath reflections and rotation of polarization. Thus, an antenna in such devices should be sensitive to both horizontally and vertically polarized waves.
Wire antennas, such as whips and helical antennas are sensitive to only one polarization direction. As a result, they are not optimal for use in portable communication devices which require robust communications even if the device is oriented such that the antenna is not aligned with a dominant polarization mode. One solution is to use microstrip patch antennas, which are capable of generating linearly polarized radiation, as well as two orthogonal modes of polarized radiation, as is the case for circularly polarized energy. For a general discussion of Microstrip Antennas including general design parameters and performance characteristics, see Pozar, D., “Microstrip Antennas, including general design parameters and performance characteristics, see Pozar, D., “Microstrip Antennas,” Proceedings of the IEEE, Vol.80, No.1, January 1992, pages 79-91, the entire contents of which being incorporated herein by reference.
Microstrip patch antennas are resonant radiating structures that can be printed on circuit boards. By feeding a number of these elements arranged on a planar surface, in such a way that their excitations are all in phase, a reasonably highly efficient antenna can be obtained that occupies a very small volume by virtue of being flat. Microstrip antennas do have some limitations, however, that reduce their practical usefulness. In general, microstrip antennas are known for their advantages in terms of lightweight, flat profiles, and compatibility with integrated circuits. A microstrip patch antenna comprises a dielectric sandwiched between a conductive ground plane and a planar radiating patch. Thus, microstrip patch antennas are useful alternatives for applications requiring a small and particularly thin overall size.
Patch antennas are commonly produced in half wavelength sizes, in which there are two primary radiating edges parallel to one another. It is known that the size may be further reduced if all of one of the primary radiating edges of a microstrip patch antenna is short circuited, permitting the size of the radiating patch to be reduced to a quarter wavelength. Additionally, it is known that the size may be reduced even further, to approximately one third the size of a half-wavelength antenna, if one of the primary radiating edges is partially shorted circuited. The short circuit is typically created by wrapping a thin sheet of copper foil to electrically connect the ground plane to the radiating patch. To simplify the manufacture of these antennas, shorting posts have been used in lieu of copper foil.
However, microstrip patch antennas are resonant structures with a relatively small bandwidth of operation and, therefore, are not optimal for wide bandwidth applications, such as data communications. It is known to improve the bandwidth of a rectangular patch antenna by placing non-driven, parasitic, patches parallel to the nonradiating edges of the driven patch.
FIG. 1 shows a typical quarter wavelength microstrip antenna 100. The antenna includes a dielectric layer 110 sandwiched between a conductive ground plane 120 and a conductive radiating patch 130. The radiating patch 130 is energized by a connection through a coaxial cable 150 to feed point 160. In microstrip antennas of this type, the length L and the width W of the radiating patch 130 are adjusted in a manner well known to those skilled in the art to achieve a desired resonant frequency.
Despite the fact that microstrip antennas have many advantages over other conventional antennas, implementation of patch antennas in wireless communications at low frequencies has been limited because the antenna becomes too large in practical applications as the frequency decreases. The length of a typical microstrip antenna has to be about half a wavelength in the substrate dielectric medium. It is known that to improve the bandwidth of a rectangular patch antenna it is possible to place non-driven, parasitic, patches parallel to the nonradiating edges of the driven patch. Although a simple alteration of the microstrip patch with symmetric sharp width discontinuities reduces the antenna size drastically, antenna efficiency, however, suffers as the antenna becomes small.